Embodiments of the present invention related to the general field of tomosynthesis.
A radiographic image is a projection of an object of interest. It is generally obtained by placing the object between an X-ray emitting source and an X-ray detector, so that the rays reach the detector after passing through the object. The radiographic image is then constructed from the data provided by the detector, and is an image of the object of interest projected onto the detector in the direction of the X-rays.
In this image, an experienced radiologist is able to distinguish radiological signs indicating a potential problem e.g. microcalcifications or opacities in mammography.
However, in a 2D projection image, the superimposition of tissues may hide lesions and under no circumstances can their actual position in the object of interest be known, since the radiologist has no information on the position of the radiological sign in the direction of projection.
Tomosynthesis has recently been developed to meet these problems; it allows a 3D image of an object of interest to be obtained in the form of a series of successive slices. These slices are reconstructed from projections of the object of interest from different angles. To do so, the object of interest is generally placed between an X-ray emitter source and an X-ray detector. The source and/or the detector are mobile so that the direction of the projection of the object onto the detector may vary (in mammography typically over an angle range of the order of 30° or more).
By looking at the tomosynthesis slices of an object of interest, a practitioner is therefore able not only to detect radiological signs in the object of interest but also to evaluate their 3D position.
Different techniques for reconstructing 3D images have been proposed allowing good quality reconstructed slices to be obtained with short computing times.
In particular, techniques using filtered backprojections (FBPs) have recently been proposed in patent US 2011/0150178 for example, or in the article:
“Generalized Filtered Backprojection Reconstruction in Breast Tomosynthesis”—Bernhard E. H. Claus, Jeffrey W. Eberhard, Andrea Schmitz, Paul Carson, Mitchell Goodsitt & Heang-Ping Chan, International Workshop on Digital Mammography, IWDM 2006, pp 167-174.
These techniques, in the projection images, particularly make use of high-pass filters which enhance the contrast of the structures. They provide image reconstruction of quality similar to that of the 3D images obtained with reconstruction techniques of iterative type, whilst allowing a limited number of iterations.
Three objects are illustrated in FIG. 1, one GS is of a large size and the two others, PS1 and PS2, correspond to structures of a smaller size, the object PS1 being more contrasted than the object PS2.
As illustrated in FIG. 1, it may happen with these techniques that contrast artefacts can be seen in the reconstructed images obtained, these propagating in the direction corresponding to the scanning of the X-ray beam when the source switches from one acquisition position to another. As can be seen by comparing the artefacts F of the beams generated in the image by the object PS1 and by the less contrasted object PS2, the more an object is contrasted the larger the artefacts and the more they are visible. In addition, the edge of the breast may at times appear to be highlighted which may be confused with an indirect sign of a lesion thereby inducing radiographer error.
Also, for interventional applications (biopsy, stereotactic surgery, etc.) highly contrasted metallic instruments may cause reconstruction artefacts that it is sought to reduce.